In announcing the investigation, Rockefeller's office said the monthly credit card fees "appear to be from a group of marketing companies that acquire consumers' billing information through agreements with popular online retail sites," which they said included Fandango.com and Orbitz.com.

Rockefeller included examples of the practice, in which, after a consumer busy something online, a pop-up appears offering cash back rewards if they sign up for an online membership service. He included an example on the committee Web site.

""The economy is hurting so many families today and we need to provide them as much relief as possible," the senator said in announcing the investigation. "Thousands of American consumers have been complaining about these deceptive practices and asking for answers - and rightly so," Rockefeller said. "There could be many more affected by these hidden mystery charges - we must do all we can to protect the hard working Americans relying on us to look out for their wallets and well-being."

Senator Rockefeller also said he had sent the following letters to what he described as "two of the leading 'post-transaction' e-commerce marketing companies," Webloyalty.com, and Vertrue Inc. giving them until June 17 to provide the committee answers to a bunch of questions.

Rockefeller says thousands have complained that they did not authorize the regular charges on their cards.

Online marketing has become a hot-button issue on Capitol Hill as Congress works on legislation to require more and more obvious consumer input on where and how their surfing information is being used.

Webloyalty, for one, says it thinks it has done nothing wrong.

"We have received the letter from the committee and they are requesting information about our marketing practices," says Beth Kitchener, VP of corporate communications for Webloyalty. "We have reviewed the letter and comments made by Senator Rockefeller and intend to cooperate fully with the committee." But she also said that the company "strongly believes that our marketing practices are clear and conspicuous and meet all the relevant standards. We also believe that that the company has conducted itself appropriately in all respects."